1. Field of the Invention
The present invention relates to the production of clean, stoichiometric, and well-ordered CdTe surfaces for the purpose of subsequent growth and/or processing of II-VI materials.
2. Description of Related Art
Cadmium telluride (CdTe) is a desirable substrate for mercury-based II-VI alloy and superlattice growth because of its close lattice match to HgTe and therefore to all HgCdTe alloy compositions. As is well-known, HgCdTe alloy composites are widely used as infrared detectors.
The performance of HgCdTe infrared detectors depends strongly upon the structural and chemical properties of the HgCdTe sensing layer, which in turn depends on the quality of the underlying substrate.
However, CdTe substrates are typically not available in the same high quality as GaAs or Si. In addition, wet chemical etching of a CdTe surface leads to non-stoichiometric, atomically rough surfaces which make poor templates for subsequent growth. The state of the CdTe surface following etching may vary considerably from substrate to substrate, because the CdTe surface appears to be extremely sensitive to the parameters of etching used during etching; these parameters include etchant concentration, temperature, time, and agitation. There are numerous conflicting reports in the technical literature regarding the stoichiometry and composition CdTe surfaces which were reported to have been subjected to the same etching procedures. Therefore, an additional treatment is required to produce a stoichiometric and ordered surface prior to the growth of HgCdTe.
There are several ways of improving the poor quality of the CdTe substrate surfaces which result from wet chemical etching. It is possible, for example, to remove contaminants from the CdTe surface by ion bombardment, followed by a thermal anneal to remove the sputter damage. Contaminants such as carbon, oxygen, chlorine, and sulfur result from exposure of the substrate surface to the ambient. Sputter damage is well-known to occur during sputtering.
We attempted to prepare a CdTe (100) substrate using very mild sputtering conditions (100 eV Ar ions at &lt;100 MA/cm.sup.2 beam current density for 5 minutes), followed by a 5 minute anneal at 300.degree. C. Auger measurements indicated that following this treatment, the surface was clean and stoichiometric. However, RHEED (reflection high-energy electron diffraction) analysis was spotty and indicative of rough surface morphology.
Alternately, it is common to grow a CdTe buffer layer on top of the CdTe substrate in an attempt to bury defects and impurities at the substrate-buffer layer interface. However, the quality of the buffer layer can also be affected by the presence of impurities and roughness on the substrate. Improvements in the quality of the substrate will result in improved buffer layer properties.
In a previous publication which includes the present inventors as authors, it was shown that compositional deviations from stoichiometry of a (100) CdTe surface resulting from exposure to 248 nm (KrF) excimer laser radiation were reversible under appropriate fluence conditions. However, improving the stoichiometry does not necessarily result in improvement of the surface structure of the CdTe substrate.
A process which improves the quality of the CdTe substrate, and hence the interface between the substrate and HgCdTe layer epitaxially grown thereon, would greatly improve the performance and reliability of the infrared detectors.